If that is an acceptable price to pay for the game, then it's yours to claim. The storefront, which is owned by CD Projekt Red, is trying to entice users to install its native client, which updated to a 2.0 version last year. The Witcher: Enhanced Edition can now be yours on PC for free by simply installing the latest version of the GOG Galaxy client.
Now's your chance to pick up The Witcher's first adventure for free.